linux-kernel - Re: [RFC PATCH for 4.18 1/2] rseq: validate rseq_cs fields are < TASK

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Date:   Thu, 28 Jun 2018 13:22:38 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
        Ben Maurer <bmaurer@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Joel Fernandes <joelaf@...gle.com>
Subject: Re: [RFC PATCH for 4.18 1/2] rseq: validate rseq_cs fields are < TASK_SIZE

On Thu, Jun 28, 2018 at 9:23 AM, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
> Validating the abort_ip field of rseq_cs ensures that the kernel don't
> return to an invalid address when returning to userspace after an abort.
> I don't fully trust each architecture code to cleanly deal with invalid
> return addresses.
>
> Validating the range [ start_ip, start_ip + post_commit_offset ] is an
> extra validation step ensuring that userspace provides valid values to
> describe the critical section.
>
> If validation fails, the process is killed with a segmentation fault.
>
> Change the rseq ABI so rseq_cs start_ip, post_commit_offset and abort_ip
> fields are seen as 64-bit fields by both 32-bit and 64-bit kernels rather
> that ignoring the 32 upper bits on 32-bit kernels. This ensures we have
> a consistent behavior for a 32-bit binary executed on 32-bit kernels and
> in compat mode on 64-bit kernels.

This is okay with me for a fix outside the merge window.  Can you do a
followup for the next merge window that fixes it better, though?  In
particular, TASK_SIZE is generally garbage.  I think a better fix
would be something like adding a new arch-overridable helper like:

static inline unsigned long current_max_user_addr(void) { return TASK_SIZE; }

and overriding it on x86 as something like:

static inline unsigned long current_max_user_addr(void) {
#ifdef CONFIG_IA32_EMULATION
  return user_64bit_mode(current_pt_regs()) ? TASK_SIZE_MAX : (1UL << 32) - 1;
#else
  return TASK_SIZE_MAX;
}

TASK_SIZE really needs to die.